This invention relates to a luminescent device and more particularly relates to such a device using a luminescent ceramic material in the form of a luminescent screen.
Zinc sulfide phosphor material has hitherto been used widely in many kinds of luminescent devices and has been supplied in powdery form or thin film.
For example, an electroluminescent device using a powder zinc sulfide phosphor is constructed in such a manner that a zinc sulfide powder is dispersed in an organic embedding resin, the dispersion is coated on a glass plate provided with a transparent electrode of such material as tin oxide to form thereon a phosphor layer, and a metal electrode is further superimposed. Such a device is disadvantageous in that because a dispersing medium is employed, the efficiency of light emission is low and the useful life is short.
An electroluminescent device using a thin film phosphor is structurally complicated in that it comprises, in a laminar arrangement, a glass substrate, a transparent electrode, a dielectric layer, a zinc sulfide thin film, a dielectric layer and a metal electrode. Therefore, its manufacture requires a high degree of skill and a high production cost. It is further disadvantageous in that high operating voltage is required. Thus the prior art device can be used only in very limited applications.
We previously found a luminescent body which is composed of a zinc oxide sintered body and a zinc sulfide luminescent layer formed on the zinc oxide body by sulfurizing the zinc oxide (U.S. Pat. No. 4,035,819). This luminescent device is simple and quite easy to manufacture. Further it does not require high operating voltage and exhibits high brightness.
In such device, however, the zinc oxide body has dark body color because luminescent impurities are also contained therein, and so emitted light can come out only through the zinc sulfide layer. Further, when this device is used as an electroluminescent device, a transparent electrode made of expensive material such as tin oxide or gold is necessarily attached to the zinc sulfide layer.